inherited membership co-opted by new signup

membership signup contribution pages support the idea of "upsells" – if a user changes their membership type during signup it will change their existing membership rather than create a new membership record. this becomes a problem if a person's existing membership is received via inheritance. to reproduce:

1. create a membership type using employer inheritance and one that does not
2. create an individual and organization record. give the organization the membership supporting inheritance. this grants the individual membership via inheritance.
3. create a contribution page with membership enabled and expose the non-inheritance membership type
4. visit the contrib page and have the logged in individual signup for that membership

this has the following effects:

1. contribution is attached to the individual (correct)
2. inherited membership on the indiv record will be changed to the new membership type (problem)

the inherited membership is still linked to the owner membership, so any changes there will flip the inherited record back to it's original type. further – inherited memberships should never be connected with a payment record.

in this scenario, I don't believe the "upsell" featured should be used. rather, a new membership record should be created.